1. Field of the Invention
The invention relates to a bearing ring for a roller bearing, a roller bearing, and a power transmission device.
2. Description of Related Art
A tapered roller bearing has characteristics that a load capacity is large and rigidity is high compared to the other rolling bearings of the same size. A tapered roller bearing is disclosed in Japanese Utility Model Application Publication No. 4-90721 (JP 4-90721 U), for example. FIG. 5 is a partial cross-sectional view showing a tapered roller bearing 51 of a related art. As shown in FIG. 5, in the tapered roller bearing 51, an inner ring raceway surface 52a on which a tapered roller 53 rolls, and a cone back face rib (hereinafter referred to as a large rib) portion 54, disposed on one side of the inner ring raceway surface 52a in the axial direction, are formed on an outer circumference of the inner ring 52. A large end face (roller end face) 53a of the tapered roller 53 slidably contacts an end face (large rib portion end face) 54a of the large rib portion 54, which is on the inner ring raceway surface 52a side. A grinding undercut portion (hereinafter referred to as a recessed portion) 55 for facilitating the grinding processing of the large rib portion end face 54a by a rotary grindstone is formed in a corner between the inner ring raceway surface 52a and the large rib portion end face 54a so as to be recessed over the entire circumference.
In the tapered roller bearing 51, a contacting position (contacting height) of the roller end face 53a having a convex curved surface and the large rib portion end face 54a having a concave curved surface in a radial direction is preferably positioned (lowered) on a radially inner side as much as possible (see arrow in FIG. 5) so that a friction of the roller end face 53a and the large rib portion end face 54a is small.
A contacting surface of the roller end face 53a and the large rib portion end face 54a is an ellipse (contact ellipse). The contact ellipse overlaps an edge (end edge) 55a of the recessed portion 55, which is on the large rib portion end face 54a side, if the contacting height is low as described above. The roller end face 53a may contact the edge 55a, which may cause an edge load and shorten the life of the tapered roller bearing 51. In particular, the edge load easily occurs when a high axial load acts on the tapered roller bearing 51 and the contact ellipse becomes large. Suppressing the edge load requires forming the recessed portion 55 small.
The edge load, however, may occur as will be described below even if the recessed portion 55 is formed small. FIG. 6A and FIG. 6B are each an explanatory view showing a production step of the inner ring 52 of the related art. FIG. 6C is an explanatory view showing a partial cross-section of the produced inner ring 52. As shown in FIG. 6A, in the production of the inner ring 52 of the related art, the recessed portion 55 is formed in an inner ring blank 57 for forming the inner ring 52. Next, as shown in FIG. 6B, the grinding processing is carried out by the outer circumferential surface of a disc-shaped rotary grindstone 58 as a finish machining process on a processing surface 54af of the large rib portion 54. The surface obtained by the finish machining process is the large rib portion end face 54a. 
In this case, if the recessed portion 55 is formed small, the outer circumferential portion of the rotary grindstone 58 cannot enter inside the recessed portion 55. Thus, as shown in FIG. 6C, a non-ground portion 60, where the grinding processing is not carried out by the rotary grindstone 58, is formed on the inner circumferential side of the large rib portion end face 54a. Accordingly, a step 62 is formed at a boundary of the non-ground portion 60 and a ground portion 61, where the grinding processing is performed by the rotary grindstone 58. Thus, if the contacting height is made low, the roller end face 53a may contact a step 62 side corner (edge) 60a of the non-ground portion 60, and the edge load may be caused. In FIG. 6C, the step 62 is illustrated in an exaggerated manner to facilitate the understanding, but the actual step 62 is small.